1. Field of the Invention
The present invention relates to a semiconductor light emitting device, more particularly relates to a semiconductor laser.
2. Description of the Related Art
FIG. 28 is a sectional view of an example of the configuration of a self-pulsation type semiconductor laser of the related art using a so-called a buried ridge structure.
Note that, here, a case where the self-pulsation type semiconductor laser is constituted by an AlGaAs-based material is shown.
As shown in FIG. 28, in this self-pulsation type semiconductor laser 10, an n-type GaAs substrate 11 has successively stacked on it an n-type AlGaAs cladding layer 12, an AlGaAs active layer 13, a p-type AlGaAs cladding layer 14, and a p-type GaAs cap layer 15.
The upper layer portion of the p-type AlGaAs cladding layer 14 and the p-type GaAs cap layer 15 have mesa-type stripe shapes extending in one direction.
Namely, a stripe portion 16 is constituted by these upper layer portion of the p-type AlGaAs cladding layer 14 and the p-type GaAs cap layer 15.
At the two side portions of this stripe portion 16 are buried a GaAs current narrowing layers 17. A GaAs current narrowing structure is formed by this.
On the p-type GaAs cap layer 15 and the GaAs current narrowing layers 17 is provided a p-side electrode 18 such as for example a Ti/Pt/Au electrode.
On the other hand, on the back surface of the n-type GaAs substrate 11 is provided an n-side electrode 19 such as for example an AuGe/Ni/Au electrode.
FIG. 29 is a schematic graph of the distribution of the refractive index of the self-pulsation type semiconductor laser 10 shown in FIG. 28.
Here, the distribution of the refractive index of the region in which light is guided in a direction parallel to a pn junction of the self-pulsation type semiconductor laser 10 and then perpendicular to a resonator length direction (hereinafter this direction will be referred to as a lateral direction) is shown in correspondence to FIG. 28.
As shown in FIG. 29, the self-pulsation type semiconductor laser 10 has a so-called step-like distribution of the refractive index in the lateral direction, where the refractive index n1 in a part corresponding to the stripe portion 16 is high and the refractive index n2 in the part corresponding to the two sides of the stripe portion 16 is low.
By changing the refractive index in steps in the lateral direction in this way, the light is guided in the lateral direction in the self-pulsation type semiconductor laser 10.
In this case, the difference xcex94n (=n1-n2) in the refractive indexes between the part corresponding to the stripe portion 16 and the parts corresponding to the two sides thereof is set to be not more than about 0.003 and the optical confinement in the lateral direction of the AlGaAs active layer 13 is eased.
At the time of operation of the self-pulsation type semiconductor laser 10 constituted in this way, as shown in FIG. 28, a width WP of a light waveguide region 22 becomes larger than a width WG of a gain region 21 inside the AlGaAs active layer 13. The light waveguide region 22 at the outside of the gain region 21 becomes a saturable absorbing region 23.
In this self-pulsation type semiconductor laser 10, by making the change in the refractive index in the lateral direction small, the seepage of light in the lateral direction is increased. By making the interaction between the light and the saturable absorbing region 23 inside the AlGaAs active layer 13 larger, self-pulsation is realized. For this purpose, it is necessary to secure a sufficient saturable absorbing region 23.
As explained above, the self-pulsation type semiconductor laser 10 has so-called a ridge structure as shown in FIG. 30, in which saturable absorbing regions are provided at the two sides of the light waveguide inside the active layer and made to perform the self-pulsation.
In this case, as shown in FIG. 30, when the relationship of P greater than G is satisfied by making the gain region inside the active layer (width thereof defined as G) created by a spread of the current as narrow as possible and conversely setting a light waveguide spot size (width thereof defined as P) relatively large, this difference acts as the saturable absorbing region and causes the self-pulsation.
This relationship is satisfied by using the refractive index difference An of the waveguide as an intermediate guide between an index guide of about 0.005 to 0.001 and a gain guide.
FIG. 31 is a perspective view of an example of the configuration of a gain guide-type semiconductor laser of the related art, FIG. 32 is a plan view of an example of the configuration of the gain guide-type semiconductor laser of the related art, and FIG. 33 is a sectional view of an example of the configuration of the gain guide-type semiconductor laser of the related art.
As shown in the figures, this gain guide-type semiconductor laser 30 is comprised of an n-type GaAs substrate 31 on which an n-type AlGaAs cladding layer 32, an AlGaAs active layer 33, a p-type AlGaAs cladding layer 34, and a p-type GaAs cap layer 35 are successively stacked.
On the two sides of this stripe portion 36 are formed a current narrowing layers 37 given a higher resistance by ion implantation of for example B+ ions.
On the p-type GaAs cap layer 35 and the current narrowing layer 37 is provided a p-type electrode 38 such as a Ti/Pt/Au electrode.
On the other hand, on a back surface of the n-type GaAs substrate 31 is provided an n-type electrode 39 such as an AuGe/Ni/Au electrode.
In the case of this gain guide-type semiconductor laser, from a practical viewpoint, the waveguide is constituted as a tapered waveguide forming a taper with a wide stripe width at the center portion becoming narrower near the end surface as shown in FIG. 32.
Note that, in FIG. 32, L denotes the entire resonator length, 11 a taper region length, 13 a wide stripe region length at the center portion, w1 a stripe width near the end surface, and w3 a stripe width at the center portion.
In the gain guide-type semiconductor laser 30 having this configuration, at the time of operation, the current flows through the stripe portion 36 and flows into the active layer 33, but since the current narrowing layer 37 is provided, the flow of the current to the two side directions of the active layer 33 is suppressed.
As a result, a light emitting region of a predetermined width is formed and laser oscillation is carried out.
In the case of a gain guide-type semiconductor laser not provided with almost any refractive index difference An in the lateral direction, since vertical multimode oscillation is carried out, the relative returned light noise characteristic is good. Also, the electrostatic withstand voltage is high, therefore there is strong surge proofness.
In the case of this gain guide-type semiconductor laser, the noise level required for the semiconductor laser is about 1% of the amount of the returned light in terms of the value of the relative intensity noise (RIN) and about xe2x88x92120 dB to xe2x88x92125 dB at the time of an output of several mW, so the laser is suitable as a light source for a CD or other optical disc.
Summarizing the problems to be solved by the invention, the semiconductor lasers explained above suffered from the following problems.
Namely, a self-pulsation type semiconductor laser with a refractive index difference xcex94n set near 0.003 and causing self-pulsation in a lateral region Inside the active layer as mentioned above has a relatively large astigmatism difference of about 10 odd xcexcm and changes in the beam spread angle xcex8// in a parallel direction of a far field pattern (FFP) due to the optical output. As a result, there is the problem that it is difficult to apply this to the optical system of an optical disc.
Further, in the case of a not illustrated index guide-type semiconductor laser, when the beam spread angle xcex8// in the parallel direction of the FFP is widened, it is necessary to make the waveguide mode width narrower, but the waveguide mode is usually formed to be basically constant, therefore it is necessary to make it narrower over the entire region. However, this means that the light density becomes higher. So-called hole burning (HB) is apt to occur and the COD level becomes lower, therefore there is the problem that it is hard to obtain a high output operation with a high and stable kink level.
Further, since the vertical mode becomes a single mode, it is susceptible to returned light noise. In particular, where the system is used in an optical disc, it is necessary to perform high frequency modulation of several hundred MHZ. For this reason, the structure of the optical pick-up is more complex than that with a gain guide-type semiconductor laser. Further, for the general structure of the ridge structure, at least two crystal growths becomes necessary and an electrode surface flattening process is added, so the process load is increased.
Further, in the case of a gain guide-type semiconductor laser with almost no refractive index difference xcex94n, a large astigmatism difference of several tens of xcexcm and a double peak property of the direction of the beam spread angle xcex8// of the parallel direction of the FFP are apt to occur. Usually, it is not practical unless a tapered waveguide as shown in FIG. 32 is adopted.
However, the tapered waveguide is susceptible to loss, so it is difficult to achieve all of the goals of low noise, low astigmatism difference, and single peak FFP by suitably adjusting the taper shape. Further, in a gain guide of the ion implantation method, the control of the refractive index difference xcex94n is almost impossible, therefore further enhancement of the characteristic is not easy.
In order to correct a large astigmatism difference of several tens of xcexcm and make the focus spot sufficiently small, an astigmatism difference correcting optical system such as an inclined plate glass is used, but addition of such an optical system requires additional members and adjustment costs, so is not desirable.
Further, there is also a possibility that other aberration such coma will be generated, therefore it is hard to say that this is suited to the coming age of high density optical discs.
An object of the present invention is to provide a semiconductor light emitting device resistant to returned light noise, able to excellent correct or reduce astigmatism difference, and stable in oscillation even at times of high output operation.
To attain the above object, according to a first aspect of the present invention, there is provided a semiconductor light emitting device comprising a first cladding layer of first conductivity type, an active layer formed on the first cladding layer, and a second cladding layer of a second conductivity type formed on the active layer, having a center portion of the second cladding layer forming a ridge structure, and comprising a stripe-shaped current injection structure, wherein a second ridge structure is formed on the second cladding layer at the two sides of the stripe portion formed on the second cladding layer via a ridge separation portion formed with a thickness in the stacking direction of the first cladding layer, active layer, and second cladding layer thinner than that of the ridge structure, a waveguide stripe width at the center is constant, and the width in a direction perpendicular to the stacking direction of the ridge separation portion is set so as to be different between the center portion in a resonator direction and near an end surface.
According to a second aspect of the present invention, there is provided a semiconductor light emitting device comprising a first cladding layer of a first conductivity type, an active layer formed on the first cladding layer, and a second cladding layer of a second conductivity type formed on the active layer, having a center portion of the second cladding layer forming a ridge structure, and comprising a stripe-shaped current injection structure, wherein the second ridge structure is formed on the second cladding layer on at the two sides of the stripe portion formed on the second cladding layer via a ridge separation portion formed with a thickness of the stacking direction of the first cladding layer, active layer, and second cladding layer thinner than that of the ridge structure, the waveguide stripe width at the center is constant, and the width in the direction perpendicular to the stacking direction of the ridge separation portion is set so as to be narrow at the center portion in the resonator direction and broader than that at the center portion near the end surface.
According to a third aspect of the present invention, there is provided a semiconductor light emitting device comprising a first cladding layer of a first conductivity type, an active layer formed on the first cladding layer, and a second cladding layer of a second conductivity type formed on the active layer, having a center portion of the second cladding layer forming a ridge structure, and comprising a stripe-shaped current injection structure, wherein the second ridge structure is formed on the second cladding layer at the two sides of the stripe portion formed on the second cladding layer via the ridge separation portion formed with a thickness in the stacking direction of the first cladding layer, active layer, and second cladding layer thinner than that of the ridge structure, the waveguide stripe width at the center is constant, and the width in the direction perpendicular to the stacking direction of the ridge separation portion is set so as to be wide at the center portion in the resonator direction and narrower than that at the center portion near the end surface.
Preferably, in the above aspects of the present invention, at least a recessed portion of the second cladding layer forming the ridge separation portion has a current narrowing structure in which a current narrowing layer of the first conductivity type is buried.
Further, preferably, an insulating film is formed in at least one part of an upper surface of the second cladding layer except a top surface of the second cladding layer forming the ridge structure at the center portion.
Still further, preferably, a current injection use cap layer is formed on the top surface of the second cladding layer forming the ridge structure at the center portion and in one part of the top surface of the second cladding layer forming the second ridge structure.
According to a fourth aspect of the present invention, there is provide a laser semiconductor light emitting device comprising a first cladding layer of a first conductivity type, an active layer formed on the first cladding layer, and a second cladding layer of a second conductivity type formed on the active layer, having a center portion of the second cladding layer forming a ridge structure, and comprising a stripe-shaped current injection structure, wherein a waveguide mechanism comprises an index guide area formed at one of a position near a light emitting front end surface for emitting a laser beam and a position near a rear end surface and having a built-in refractive index difference for the waveguide in a lateral direction perpendicular to a resonator length direction and a gain guide area formed in the region except the index guide area and not having the built-in refractive index difference.
In this aspect of the present invention, preferably, in the index guide area, the index guide mechanism is constituted so that the refractive index difference is gradually increased over a range from a connection portion with the gain guide region to the light emitting front end surface.
Further, preferably, the index guide mechanism is constituted by grooves formed on the two sides of the stripe portion and formed so that the groove width thereof gradually becomes larger over a range from the connection portion with the gain guide area to the light emitting front end surface.
Alternatively, preferably, the index guide mechanism is constituted by grooves formed on the two sides of the stripe portion and formed so that a distance from a bottom surface of the groove to the active layer gradually becomes smaller over the range from the connection portion with the gain guide area to the light emitting front end surface.
Alternatively, preferably, the index guide mechanism is constituted by grooves formed on the two sides of the stripe portion and formed so that the groove width gradually becomes larger over the range from the connection portion with the gain guide area to the light emitting front end surface and then the distance from the bottom surface of the groove to the active layer gradually becomes smaller over the range from the connection portion with the gain guide area to the light emitting front end surface.
Further, preferably, the stripe portion forms a taper shape with a width which is wide at the center portion and becomes narrow near the end surface.
Alternatively, preferably, the stripe portion forms a schematically uniform straight shape over the entire width thereof.
According to a fifth aspect of the present invention, there is provided a semiconductor light emitting device comprising a first cladding layer of a first conductivity type, an active layer formed on the first cladding layer, and a second cladding layer of a second conductivity type formed on the active layer and comprising a stripe-shaped current injection structure, wherein a current noninjection portion is formed at the center region of the stripe portion.
According to a sixth aspect of the present invention, there is provided a semiconductor light emitting device comprising a first cladding layer of a first conductivity type, an active layer formed on the first cladding layer, and a second cladding layer of a second conductivity type formed on the active layer and comprising a stripe-shaped current injection structure, further comprising layers which absorb the light at positions asymmetrical with respect to the active layer in an optical confinement mode in the stacking direction of the first cladding layer, active layer, and second cladding layer.
Preferably, in this aspect of the invention, preferably the layers absorbing the light are first and second absorption layers formed at positions asymmetrical with respect to the active layers in the first cladding layer of first conductivity type and the second cladding layer of second conductivity type.
Further, preferably, provision is made of a semiconductor substrate having the first cladding layer formed in a front surface region and a cap layer formed on the second cladding layer, and wherein the thicknesses of the first and second cladding layers are set to values so that at least the skirts of the optical confinement mode in the vertical direction reaches the semiconductor substrate and the cap layer, and the semiconductor substrate and the cap layer are the layers absorbing the light.
According to the present invention, for example, in the case of a self-pulsation type semiconductor laser in which the waveguide stripe width at the center is constant and the width in the direction perpendicular to the stacking direction of the ridge separation portion are set narrow at the center portion in the resonator direction and broader than that at the center portion near the end surface, the spread of the light in the lateral direction becomes larger than the stripe width of the second cladding layer. Namely, at the center waveguide portion. the light spreads in the lateral direction, but the current remains narrow as it is, therefore a saturable absorbing region effective for pulsation is sufficiently obtained. By this, a state where the light is focused like with a so-called index guide and the pulsation is no longer generated does not occur, and the pulsation is stably continuously generated.
On the other hand, near the end surface, the light mode is focused at the center and the effective refractive index difference xcex94n becomes larger, therefore the result becomes close to that of an index-type waveguide. By this, the astigmatism difference is corrected, and the beam spread angle xcex8// in the parallel direction of FFP is spread.
Further, in the case of a semiconductor laser in which the waveguide stripe width at the center is constant and the width in the direction perpendicular to the stacking direction of the ridge separation portion is set narrow at the center portion in the resonator direction and broader than that at the center portion near the end surface, the waveguide mode becomes narrow at the center portion and becomes wide near the end surface.
In a material causing large deterioration at the end surface or a high output laser having a high end surface optical density, it is necessary to lower the optical density of the end surface for securing the reliability thereof.
For this reason, a flaring structure in which the stripe width is increased near the end surface has been used, but it becomes possible to achieve an effect equivalent to this while keeping the shape of the straight stripe as it is by the present structure.
Further, according to the present invention, since there is a waveguide mechanism in which the index guide area is connected to the gain guide area, the astigmatism difference is reduced while making good use of the vertical mode property of the gain guide, that is, while securing the low noise characteristic strong against the returned light as it is, and the focus spot system becomes small.
Further, since the invention is designed so that the refractive index difference is gradually increased over the range from the connection portion with the gain guide area to the light emitting front end surface in the index guide area, the waveguide surface of the gain guide area, that is, the curved wave surface, gradually changes to a plane wave surface in the index guide area. The wave surface is gently changed, and the astigmatism difference is corrected inside the laser while reducing the energy loss.
Further, according to the present invention, in a semiconductor laser in which the current noninjection portion is formed at the center region of the stripe portion, the waveguide loss becomes large at the center region of the stripe portion in which the current noninjection portion is formed.
Accordingly, since the waveguide surface will be curved with respect to the direction of advance of light, a delay occurs between the stripe center portion having a large loss and the two end portions of the stripe.
Further, in a semiconductor laser provided with light absorption layers, the wave surface in the vertical direction is corrected from a recessed wave surface to a convex wave surface.